Microsoft Security Bulletin (MS00-086) Patch Available for Web Server File Request Parsing Vulnerability Originally posted: November 06, 2000 Updated: November 30, 2000 Summary On November 06, 2000, Microsoft released the original version of this bulletin, announcing the availability of a patch that eliminates a security vulnerability in Microsoft® Internet Information Services 5.0. The vulnerability could enable a malicious user to run operating system commands on a web server. Since its original issuance, the bulletin has been updated several times: * On November 10, 2000, the bulletin was updated to clarify the scope of the issue. * On November 21, 2000, it was updated to discuss two newly-discovered variants of the original vulnerability. * On November 30, 2000, it was updated to discuss a newly-discovered regression error in the IIS 5.0 patch and recommend that customers apply an updated version of the patch. The newly-discovered regression error only affects the IIS 5.0 version of the patch. It has no effect on the effectiveness of the patch against the vulnerability discussed here, but it does cause servers to be vulnerable to the Web Server Directory Traversal discussed in Microsoft Security Bulletin MS00-078, even if the patch provided in MS00-078 has been applied. Microsoft therefore recommends that all IIS 5.0 customers apply the new patch provided below. It protects against both the Web Server File Request Parsing and Web Server Directory Traversal vulnerabilities. The IIS 4.0 version of the patch does not contain the error, and customers who have applied the IIS 4.0 patch do not need to take any action. Frequently asked questions regarding this vulnerability and the patch can be found at http://www.microsoft.com/technet/security/bulletin/fq00-086.asp Issue When IIS receives a valid request for an executable file, it passes the name of the requested file to the underlying operating system for processing. However, due to an implementation flaw, it is possible to create a specially-malformed file request that contains both a file name and one or more operating system commands. Upon receiving such a request, IIS would pass the entire string to the operating system, which would first process the file and then execute the commands. The ability to execute operating system commands on the web server would enable a malicious user to take virtually any action that an interactively-logged on user could take. Although this would not give the malicious user administrative control over the server, it would nevertheless enable him to cause widespread damage. He could, for instance, add, delete or change files on the server, run code that was already on the server, or upload code of his choice and run it. There are three significant restrictions on type of file request that could be used to exploit this vulnerability: * The malicious user would need to request a .bat or .cmd file. * The file would need to exist. * The malicious user would need to have execute permissions on the file. Although these restrictions limit the scope of the vulnerability, it is important not to discount it. Many third-party software products for web servers install batch files by default. As a result, Microsoft recommends that all customers running affected versions of IIS verify whether their systems contain any .bat or .cmd files that can be executed by visitors to the site, and apply the patch immediately if this is the case. The patch for this issue also eliminates the Web Server Directory Traversal vulnerability discussed in Microsoft Security Bulletin MS00-078. Affected Software Versions * Microsoft Internet Information Server 4.0 * Microsoft Internet Information Services 5.0 Patch Availability * Internet Information Server 4.0: http://www.microsoft.com/ntserver/nts/downloads/critical/q277873 * Internet Information Services 5.0: http://www.microsoft.com/Downloads/Release.asp?ReleaseID=25547 Note: The IIS 5.0 patch can be applied atop systems running either Windows 2000 Gold or Service Pack 1. It will be included in Windows 2000 Service Pack 2. Note: The IIS 4.0 patch can be applied atop systems running Windows NT 4.0 Service Pack 6a. It will be included in Windows NT 4.0 Service Pack 7. Note Additional security patches are available at the Microsoft Download Center More Information Please see the following references for more information related to this issue. * Frequently Asked Questions: Microsoft Security Bulletin MS00-086, http://www.microsoft.com/technet/security/bulletin/fq00-086.asp * Microsoft Knowledge Base (KB) article Q277873, http://www.microsoft.com/technet/support/kb.asp?ID=277873 * Microsoft TechNet Security web site, http://www.microsoft.com/technet/security/default.asp Obtaining Support on this Issue This is a fully supported patch. Information on contacting Microsoft Product Support Services is available at http://support.microsoft.com/support/contact/default.asp. Acknowledgments Microsoft thanks NSFocus (http://www.nsfocus.com) for reporting the original and new variants of this vulnerability to us and working with us to protect customers. Revisions * November 06, 2000: Bulletin Created. * November 10, 2000: Bulletin updated to indicate that IIS 4.0 is affected when running on pre-SP6 versions of Windows NT 4.0, and to provide information on additional restrictions on the vulnerability. * November 21, 2000: Bulletin updated to discuss availability of patch that addresses new variants of vulnerability. * November 30, 2000: Bulletin updated to discuss regression error and recommend that customers apply updated patch. THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY. [spacer.gif] Send this document to a colleague [spacer.gif] [spacer.gif] Printer-friendly version Last updated November 30, 2000 © 2000 Microsoft Corporation. All rights reserved. Terms of use.